This section provides background information related to the present disclosure which is not necessarily prior art.
High-energy density, electrochemical cells, such as lithium ion batteries can be used in a variety of consumer products including personal electronics, power tools, lawn and garden equipment, motorized wheelchairs, toys, and for transportation. An exemplary use in transportation includes vehicles, such as Hybrid Electric Vehicles (HEVs) and Electric Vehicles (EVs). Typical lithium ion batteries comprise a first electrode (e.g., a cathode), a second electrode (e.g., an anode), an electrolyte material, and a separator. Often a stack of lithium ion battery cells is electrically connected to increase overall output. Conventional lithium ion batteries operate by reversibly passing lithium ions between the negative electrode and the positive electrode. A separator and an electrolyte are disposed between the negative and positive electrodes. The electrolyte is suitable for conducting lithium ions and may be in solid or liquid form. Lithium ions move from a cathode (positive electrode) to an anode (negative electrode) during charging of the battery, and in the opposite direction when discharging the battery. For convenience, a negative electrode will be used synonymously with an anode, although as recognized by those of skill in the art, during certain phases of lithium ion cycling, the anode function may be associated with the positive electrode rather than the negative electrode (e.g., the negative electrode may be an anode on discharge and a cathode on charge).
Contact of the anode and cathode materials with the electrolyte can create an electrical potential between the electrodes. When electron current is generated in an external circuit between the electrodes, the potential is sustained by electrochemical reactions within the cells of the battery. Each of the negative and positive electrodes within a stack is connected to a current collector (typically a metal, such as copper for the anode and aluminum for the cathode). During battery usage, the current collectors associated with the two electrodes are connected by an external circuit that allows current generated by electrons to pass between the electrodes to compensate for transport of lithium ions.
Many different materials may be used to create components for a lithium ion battery. By way of non-limiting example, cathode materials for lithium batteries typically comprise an electroactive material which can be intercalated with lithium ions, such as lithium-transition metal oxides or mixed oxides of the spinel type, for example spinel LiMn2O4, LiCoO2, LiNiO2, LiMn1.5Ni0.5O4, LiNi(1−x−y)CoxMyO2 (where 0<x<1, y<1, and M may be Al, Mn, or the like), or lithium iron phosphates. The electrolyte typically contains one or more lithium salts, which may be dissolved and ionized in a non-aqueous solvent. The negative electrode typically includes a lithium insertion material or an alloy host material.
Typical electroactive materials for forming an anode include lithium-graphite intercalation compounds, lithium-silicon intercalation compounds, lithium-tin intercalation compounds, and lithium alloys.
Many conventional Li-ion batteries can suffer from capacity fade attributable to many factors, including the formation of passive film known as solid electrolyte interphase (SEI) layer over the surface of the negative electrode (anode), which is often generated by reaction products of anode material, electrolyte reduction, and/or lithium ion reduction. Active material (Li+) in the positive electrode can thus form a stable film on surfaces of the electrodes. During the formation and initiation period, active lithium is lost as it passivates the negative electrode. This irreversible reaction, observed in the initial charging cycle, leads to the formation of a stable passive film on the anode. Hence, a portion of an initial amount of lithium present in a battery cell is incorporated into the formation of the SEI resulting in a high irreversible capacity loss (IRCL). The SEI layer formation plays a significant role in determining electrode behavior and properties including cycle life, irreversible capacity loss, high current efficiency, and high rate capabilities, particularly advantageous for power battery and start-stop battery use. Typically, 10-20% of the lithium present in the battery is irreversibly lost during this initial cycle.
To compensate for initial loss of lithium (e.g., in the SEI), extra lithium capacity may be incorporated onto the positive electrode of the battery. However, this approach reduces the energy density of the battery and potentially leads to undesirable lithium plating on the negative electrode. It would be desirable to develop methods of forming high power lithium ion batteries with materials that counteract initial loss of lithium capacity to minimize capacity fade and maximize charge capacity for long-term use.